The present invention relates to an asynchronous machine for a starter-generator of an internal combustion engine.
Asynchronous machines of this type are of interest for many uses, such as, for example, machine tools, electric tools, or in the motor vehicle sector for hybrid drives, electrically-driven turbochargers, or starter-generators for internal combustion engines, which must be designed partly for very high maximum engine speeds.
If an asynchronous machine with a short circuit rotor is used as a starter-generator, in which the short circuit cage, or “squirrel cage” of the rotor is made as one-piece from aluminum die casting, the standards based on performance and efficiency frequently are not fulfilled, on account of which pure copper is used not infrequently as the cage material. This squirrel cage made of pure copper comprises generally cold-formed copper bars, which are inserted in receiving grooves of the stack of sheets of the rotor and are hard-soldered on their front ends, respectively, with one of two short circuit rings, which generally are either cast from copper or are stacked from multiple stamped copper sheets. As a result of the soldering of the short circuit bars with the short circuit rings, however, the physical properties of the copper material that is utilized are drastically decreased in the area of the solder points and around these points. As a result, with high speeds, the elastic limit of the material is exceeded, which leads to plastic deformation. This deformation takes the form of bowing out or warping, which begins in the inner diameter of the short circuit rings, and in extreme cases, can lead to tearing of the short circuit rings and to destruction of the asynchronous machine.
In order to make the rotors sufficiently speed-fixed, additional features are utilized, such as, for example, the mounting of so-called reinforcement rings made from a metal with high resistance, which encompasses the adjacent short circuit ring on its front side facing away from the stack of sheets, so that it outwardly supports the short circuit ring and absorbs a portion of the centrifugal forces acting on the short circuit ring with high speeds.
The Applicants' DE 199 55 050 A1 disdose such a reinforcement ring, which is attached with screws on the stack of sheets and has a circumferential groove open on one side in the axial direction, which accommodates the front end of the squirrel cage overlying the adjacent face surface of the stack of sheets with the short circuit ring. If this type of reinforcement ring is made from sheet metal, which has magnetic properties, this leads to a good magnetic inference for the stray field on the front end of the rotor, which results in an improper enlargement of the rotor dispersion and therewith, associated power losses of the asynchronous machine. Basically, these disadvantages could be avoided if a non-magnetic type of steel is used when manufacturing this type of reinforcement ring. However, this leads to significantly higher material costs and makes more difficult the mechanical machining of the reinforcement ring.